The notion of realization is defined so that we can better understand what it means to say that mentality is physically realized. It is generally thought that physical properties realize mental properties (thesis PR). The definitions provided here support this belief, but they also reveal that mental properties can be viewed as realizing physical properties. This consequence questions the value of PR in helping us capture the idea that mental phenomena are dependent upon (i.e., obtain by virtue of) physical phenomena. In particular, Kim's functional model of reduction and Melnyk's functional definition of physicalism are refuted.